3.4. M ULTIFUNCTIONAL P ERFORMANCE OF THE E LECTRO - OPTICALLY U NIVERSAL
3.4.2. Demonstration of Dynamic Focusing Meta-Mirror
In our full-wave electromagnetic simulations, we modeled a miniaturized lens with a 20 m aperture size since simulating the full metasurface at the small mesh sizes required for the ITO layer active region is beyond our present numerical simulation capability.
Figures 3.21d-f illustrate the far-field pattern of the beam reflected from our tunable metasurface in the x-z plane. As seen in Figs. 3.21d-f, the metasurface can clearly focus the reflected light at the focal lengths of 1.5 m, 2 m, and 3 m when appropriate bias voltages are applied to the individual metasurface pixels.
We then experimentally characterize the dynamic focusing meta-mirror once the focusing performance of our multifunctional metasurface was confirmed by calculations.
In order to measure the reconfigurable focusing performance of the metasurface, we use a part of the universal setup depicted in Fig. 3.22.
Figure 3.22: Optical setup used for focusing performance measurement. NIR-L: NIR laser, M: mirror, I: iris, BS: 50:50 beam splitter, P: polarizer, O: objective lens, MTS: metasurface sample, L: lens, IR-C: IR CCD camera, MS: 2-axis motorized stage. The components indicated with red cross marks belong to the universal setup and are not used in this part of the measurement.
In this setup, the metasurface sample is illuminated by a coherent beam from a tunable NIR laser. The laser beam is directed to the sample surface passing through an iris (I), a polarizer (P), an objective (O) with a long working distance (Mitutoyo M Plan Apo 20×, NA = 0.40), and a 50:50 non-polarizing beam splitter (BS). The reflected beam
from the metasurface is then captured by an imaging system. The imaging system consists of an IR camera (IR-C), and an objective lens (O) with long working distance (Mitutoyo M Plan Apo 20×, NA = 0.40, WD = 20 mm) paired with a tube lens. The imaging system is then moved from the image plane by a 2-axis motorized stage (MS), that could move in the x- and y-directions with a resolution of 100 nm, and the intensity profile of the reflected beam in the x-y plane is captured at different depths.
We then program the voltages applied to each metasurface pixel to experimentally achieve the desired phase shift values. Figure 3.23 shows the spatial phase profile of focusing meta-mirrors designed to have focal length of f = 1.5 µm (Fig. 3.23a), f = 2 µm (Fig. 3.23b), and f = 3 µm (Fig. 3.23c). The square points show the ideal required phase values obtained from Eq. (3.2) and the diamond points represent the phase values acquired by the metasurface obtained from phase measurements (Fig. 3.13).
Figure 3.23: Experimental demonstration of a dynamic focusing meta-mirror with short focal length. Spatial phase distribution of focusing meta-mirror with focal lengths of (a) f = 1.5 µm, (b) f = 2 µm, and (c) f = 3 µm. Square points show the ideal phase values and diamond points present the phase values acquired by the metasurface. Spatial voltage distribution of focusing meta-mirror with focal lengths of (d) f = 1.5 µm, (e) f = 2 µm, and (f) f = 3 µm.
Measured intensity profile of the beam reflected from the focusing meta-mirror with focal lengths of (g) f = 1.5 µm, (h) f = 2 µm, and (i) f = 3 µm. The scale bar is 2 μm.
After fitting the measured phase values to the ideal curves, we obtain the voltage values corresponding to the fitted phase profiles. Figures 3.23d-f show the spatial voltage profile of the focusing meta-mirrors with a focal length of f = 1.5 µm, f = 2 µm, and f = 3 µm.
Using the setup shown in Fig. 3.22, the intensity profile of the reflected beam in the x- y plane is recorded. By extracting the cross-sections of the captured intensity profiles at fixed y values, we reconstruct the intensity profile of the reflected beam in the x-z plane. Figures 3.23g-i illustrate the metasurface reflected beam intensity profiles in the x-z plane for the applied bias distributions shown in Figs. 3.23d-f.
As can be seen, the fabricated metasurface focuses the reflected beam at the desired depths. The scale bars in Figs 3.23g-i are obtained by imaging an object of known size.
When the incident light is polarized perpendicular to the antennas, no focusing is observed since no phase modulation could be achieved in that polarization. This observation confirms that the captured focusing originates from the metasurface.
Using the same concept of individually-controlled metasurface pixels, we reprogram the bias voltages applied to the metasurface in order to experimentally demonstrate a tunable focusing meta-mirrors with focal length varying from 15 m to 25 m. Figures 3.24a-c show the spatial distribution of the phase shift (diamond) and the corresponding applied bias voltage (square) required to focus the reflected beam at the focal lengths of 15 m, 20 m, and 25 m. The voltage distribution is designed using the measured bias voltage/phase relationship (Fig. 3.13). The measured intensity profiles mapped in the x-z plane are illustrated in Fig. 3.24d-f.
As can be seen, our universal metasurface is able to provide tunable focusing with reconfigurable focal lengths only by reprogramming the voltages applied to it. The focal length could be tuned from small values (1.5 m) and can be extended to large ones (25 m).
We can further increase the focal length of the metasurface by increasing the number of metasurface pixels that are individually controlled. Figure 3.25 shows the spatial phase profile and the corresponding voltage profile of the focusing metasurface with focal length as large as f = 150 µm (Fig. 3.25a, d), f = 200 µm (Fig. 3.25b, e), and f = 250 µm (Fig. 3.25c, f).
Figure 3.24: Experimental demonstration of a dynamic focusing meta-mirror with long focal length. Spatial phase distribution of focusing meta-mirror with focal lengths of (a) f = 15 µm, (b) f = 20 µm, and (c) f = 25 µm. Square points show the ideal required phase values and diamond points present the phase values acquired by the metasurface. Spatial voltage distribution of focusing meta-mirror with focal lengths of (d) f = 15 µm, (e) f = 20 µm, and (f) f = 25 µm. Measured intensity profile of the beam reflected from the focusing meta-mirror with focal lengths of (g) f = 15 µm, (h) f = 20 µm, and (i) f = 25 µm. The scale bar is 2 μm.
Figure 3.25: Possibility of demonstration of focusing meta-mirror with extended focal lengths using the multifunctional metasurface. Spatial phase distribution of focusing meta- mirror with focal lengths of (a) f = 150 µm, (b) f = 200 µm, and (c) f = 250 µm. Square points show the ideal required phase values and diamond points present the phase values acquired by the metasurface. Spatial voltage distribution of focusing meta-mirror with focal lengths of (d) f = 150 µm, (e) f = 200 µm, and (f) f = 250 µm.
The focusing meta-mirrors with the phase/voltage profiles presented in Fig. 3.25 consist of 288 individually-addressable metasurface pixels. Such tunable focusing meta-mirror with micro-scale focal length can be potentially applied in many applications, such as light-field imaging [139] and full-color imaging [140].